A noninvasive instrument for measuring oxygen saturation of blood in retinal vessel has been developed. The method is based upon the photoelectric measurement of the optical density of retinal vessels at three wavelength (568, 576 and 596 nm). Initial evaluation of the technique is presently being performed on human volunteers. In vitro measurements of the optical properties of blood circulating in glass capillary tubes are being made under conditions that simulate retinal vessels of the ocular fundus.